European and U.S. officials have denounced an arson attack on a pro-Russian Ukrainian television station, calling such violence "unacceptable."

European Commission spokeswoman Maja Kocijancic on September 6 urged Ukraine to conduct a full, independent investigation into the attack on the TV station, which was set ablaze in Kyiv over allegations that it is pro-Russian.

Five people were hurt in the fire at the Inter TV studios in Kyiv on September 4.

Kocijancic said Brussels "plays close attention to all incidents of violence against journalists and media outlets."

Washington earlier condemned as "unacceptable" the attack on Inter TV by about 20 Ukraine nationalists wielding firebombs and dressed in camouflage fatigues.

"We support thorough investigation into arson at Inter, are following closely,” the U.S. Embassy in Kyiv tweeted on September 5.

Ukrainian authorities said they detained nine people in connection with the fire, which apparently was set off by a smoke bomb thrown into the building, but that all were released after questioning.

The Organization for Security and Cooperation in Europe's media-freedom representative also strongly condemned the attack.

"Violence is never an acceptable response to disagreements with editorial policy, even if the reporting is seen as provocative and controversial," Dunja Mijatovic said.

Mijatovic said she was "encouraged" by the swift law enforcement response and the condemnation of the attack by Ukrainian President Petro Poroshenko and Ukraine's journalism union.

But she noted that other Kyiv officials and groups have accused the broadcaster of disloyalty to Ukraine and are seeking to suppress its reporters.